1. Field of the Invention
The present invention relates to a device for controlling the movement of a light spot of a light beam of a system for reading out recorded information, and more specifically to a device which controls the movement of the light spot during a period for starting an operation of a focus servo system.
2. Description of Background Information
In an optical system for recording and reading out recorded information, a read out laser beam is applied on the surface of a recording disc which is used as a recording medium. Since the recorded information is picked up by reflection of the read out laser beam or by transmitted light of the read out laser beam, it is necessary to accurately focus the light spot of the read out laser beam on the recording surface of the recording disc. Therefore, a focus servo system, which controls the position of a focal lens in accordance with a focus error signal, is generally attached to the optical recording and read out system.
In the focus servo system, the magnitude of the focus error signal becomes equal to zero when the focal lens is positioned at an in-focus position which causes the light spot passing through the focal lens to correctly converge on the recording surface of the recording disc. Assume that the distance between the focal lens and the recording surface in the above state is expressed D.sub.o, the magnitude and the sign of the focus error signal varies in such a manner that the magnitude substantially increases in proportion to the deviation of the distance from the value D.sub.o, and the sign of the focus error signal changes depending on the direction of the deviation from the in-focus position. In short, the focus error signal has a so-called S curve characteristic having a center at the distance of D.sub.o.
In accordance with this focus error signal, a motor is driven to move the focal lens up and down along a direction perpendicular to the surface of the recording disc. With this operation of the focus servo system, the focal lens is correctly moved to maintain the proper distance between the focal lens and the recording surface regardless of any deformation of the recording disc or other structural errors which cause the up and down movement of the recording surface for each revolution of the recording disc.
Furthermore, prior to the start of reading the recorded information, the focal lens is placed at a position furthest away from the recording surface, in which the distance between the focal lens and the recording surface is the greatest and larger than the value D.sub.o. In this state, the generation of the focus error signal is not possible since the light spot of the read out laser beam is not focused on the recording surface. Therefore, the focus servo system must be operated under an open loop control in which the movement of the focal lens is controlled by a signal other than the focus error signal.
Accordingly, it is necessary to provide a starting signal which moves the focal lens down until the distance between the focal lens and the recording surface becomes near the value D.sub.o. After the focal lens has approximately reached position D.sub.o, the focus servo system will be operated under a closed loop operation in which the position of the focal lens is adjusted according to the focus error signal.
However, in conventional arrangements, there is a drawback in that a circuit for generating this starting signal is rather complicated. Furthermore, the operating characteristic of the circuit is subjected to changes due to the temperature characteristics of the circuit elements. Therefore, an adjustment of the operating characteristic of this circuit is difficult and troublesome.